/*
 * Copyright 2020 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


export interface OnHiddenCallback {
  (event: Event): void;
}


export const onHidden = (cb: OnHiddenCallback, once?: boolean) => {
  const onHiddenOrPageHide = (event: Event) => {
    if (event.type === 'pagehide' || document.visibilityState === 'hidden') {
      cb(event);
      if (once) {
        removeEventListener('visibilitychange', onHiddenOrPageHide, true);
        removeEventListener('pagehide', onHiddenOrPageHide, true);
      }
    }
  }
  addEventListener('visibilitychange', onHiddenOrPageHide, true);
  // Some browsers have buggy implementations of visibilitychange,
  // so we use pagehide in addition, just to be safe.
  addEventListener('pagehide', onHiddenOrPageHide, true);
};
